In today's cellular networks, speech communication over the air interface is conveyed through circuit-switched links, or channels, that are reserved for the duration of the call. In the future, all data transfer, including voice communications, will be conveyed using packets of information transported over connectionless links. For the case of voice communications, this is known as Voice over Internet Protocol (VoIP).
A fundamental trade-off between speech quality and system capacity exists in all cellular networks. An increase in one results in a decrease in the other, other things being equal. Improvements are made when either the speech codec is made more efficient in compressing speech, resulting in higher system capacity and/or improved voice quality, or the air interface itself is improved, again resulting in higher system capacity.
Increases in system capacity and improvements in voice quality in the past have been made by independently improving the speech codec and the air interface that transports its encoded frames. As an example, in cdma2000®, the IS-127 speech codec standard has been supplanted with the IS-893 standard. Through improvements to the air interface, Revision 0 of cdma2000 significantly increased the capacity of the system over that of IS-95 and its revisions. These standards revisions reflect major upgrades to cdma2000 equipment. This invention can enable further improvements in addition to and complementary to these approaches.
The use of feedback from the decoder to the encoder, in addition to the establishment of a link between the codec and the channel that transports its output, opens the door for further optimization of the trade-off between cellular system capacity and voice quality without impacting the fundamental operation of either the speech codec or the channel that carries its encoded frames. The transition of voice communications from a circuit-switched channel to a packet-switched channel provides an opportunity for introducing these improvements.
The cdma2000 air interface standard now defines a packet data channel for use in transporting packets of data over the air interface. This packet data channel is optimized for transporting high-speed bursts of delay-insensitive data with very low probability of error through repeated retransmission if necessary. It is assumed that all packets are equally important and that all are essential.
Conversely, the attributes of data representing speech include low speed, non-burstiness, delay sensitivity and error tolerance. It is known that not all packets are perceptually equivalent. These unique attributes of speech are not taken into specific account by the design of the packet data channel. Thus, the current systems utilize extra resources and therefore reduce throughput by retransmitting packets that are not necessary.
While the cdma2000 packet data channel can be used as defined to transport speech information, improvements can be made by taking into account the specific nature of speech in addition to linking the specific application, speech coding, to the channel that transports the encoded speech.
Current cellular systems convey speech through circuit-switched connections. There is no inter-layer communication in place between the speech codec and the circuit-switched channel that transports the codec's encoded frames of information. In addition, there is no feedback from the speech decoder to the speech encoder.